The absorption coefficient for electrons has been measured in the vapors of the alkali metals, Na, K, Rb, and Cs. Measurements were made with a beam of electrons bent by a magnetic field through a series of slits arranged on a circle so as accurately to determine the velocity. The path length was kept constant while the pressure was varied by changing the temperature of the metal. The actual value of the vapor pressure is not known accurately and consequently the magnitude of the absorption coefficient is a little uncertain. The shape of the curve is independent of this uncertainty. The absorption coefficient for all the alkali metals increases monotonically from high to low velocity except in the region of the critical potentials where a sharp peak is superimposed on the monotonic rise.

• Received 27 July 1929

DOI:https://doi.org/10.1103/PhysRev.34.673